Forward error correction scheme for high rate data exchange in a wireless system

ABSTRACT

A transmitter/receiver system for high data transfer in a wireless communication system includes a physical layer processor that comprises an FEC coder, a demultiplexer and a plurality of modem processors. The FEC coder applies error correction codes to the high data rate signal. Thereafter, the demultiplexer distributes portions of the coded high data rate signal to the modem processors. Each modem processor processes its respective portion of the coded signal for transmission in an independent channel.

BACKGROUND

The present invention provides a low latency error correction mechanismfor high data rate transmissions over multiple traffic channels in awireless communication system.

It is known to include forward error correction (“FEC”) coding anddecoding to information signals that are to be transmitted over awireless channel. Forward error correction, generally speaking,introduces predetermined redundancy into an information signal to permita receiver to identify and perhaps correct errors that may have beenintroduced by a transmission medium. For example, the known IS-95standard for code division multiple access cellular communicationspecifies a type of convolutional code for each traffic channeltransmitted from base station to mobile station or vice versa.

Recently, it has been proposed to provide high data rate exchanges overa wireless communication system. Such high data rate exchanges may beused, for example, to facilitate data transfer for computingapplications or for video conferencing applications. In one suchproposal, a high rate data signal may be communicated to a receiver overa plurality of parallel traffic channels. For example, the recentlyproposed IS-95 B standard proposes use of parallel CDMA channels eachhaving a data rate of 9.6 KB/s to provide a higher data ratecommunication. In such systems, a high rate data signal is demuliplexedinto a plurality of lower rate data signals and each of these signals isprocessed in an independent traffic channel. Thus, each lower rate datasignal has FEC applied to it.

Another example of a wireless CDMA system providing multiple paralleltraffic channels for high data rate exchange may be found in thecopending patent application “Protocol Conversion and BandwidthReduction Technique Providing Multiple nB+D ISDN Basic Rate InterfaceLinks Over a Wireless Code Division Multiple Access CommunicationSystem,” Ser. No. 09/030,049 filed Feb. 24, 1998 the disclosure of whichis incorporated herein.

Wireless communication channels are inherently “noisy” due to channelimpairments caused by atmospheric conditions, multipath effects,co-channel interference and the like. Particularly if used for computingapplications, where executable content may be expected to be exchangedover traffic channels, the need for powerful FEC techniques willcontinue to be prevalent.

Use of more powerful FEC techniques in such wireless systems mayincrease the latency of data requests. For example, the known turbocodes require large blocks of data to be received entirely by a decoderbefore decoding can begin. Latency refers generally to the delay thatextends from the time a request for data is issued by a user and thetime when data responsive to the request is presented to the user. FECintroduces decoding delays at a wireless receiver and, thus, contributesto latency.

There is a need in the art for a wireless communication system thatprovides high data rate exchange having high quality FEC with lowlatency.

SUMMARY

Embodiments of the present invention provide a transmitter/receiversystem for high data transfer in a wireless communication system inwhich a physical layer processor comprises an FEC coder, a demultiplexerand a plurality of modem processors. The FEC coder applies errorcorrection codes to the high data rate signal. Thereafter, thedemultiplexer distributes portions of the coded high data rate signal tothe modem processors. Each modem processor processes its respectiveportion of the coded signal for transmission in an independent channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a transmitter and a receiver each constructed inaccordance with embodiments of the present invention.

DETAILED DESCRIPTION

The present invention provides low latency forward error correction fora high data rate wireless transmission by applying forward errorcorrection codes to data prior to multiplexing the data across aplurality of parallel fixed bandwidth traffic channels.

FIG. 1 is a block diagram of a transmitter 100 and a receiver 200 eachconstructed according to embodiments of the present invention. Thetransmitter 100 and receiver 200 are illustrated as operating in alayered communication system that includes a transmission layer (130,230), a physical layer (120, 220) and higher layer communications suchas network layers and data link layers (collectively labeled 110 and210). As is known, in the transmission layer 130, a transmitter 100performs carrier modulation, amplification and transmission upon digitaldata to be transmitted. Also as is known, in the transmission layer 230,a receiver 200 performs reception, amplification and carrierdemodulation to obtain a recovered digital data signal. The higherlayers 110, 210 of the communication system also may process aninformation signal as may be required for the application for which thepresent invention is to be used.

According to an embodiment of the present invention, the physical layer120 of the transmitter 100 may be populated by an FEC coder 122, ademultiplexer 124 and a plurality of modem processors 126 a-126 n. Thenumber of modem processors 126 a-126 n may vary and also may bedetermined by the quantity of data to be transmitted and the capacity ofeach of the traffic channels over which the data may be transmitted. TheFEC coder 122 receives a source signal from a higher layer 110 in thetransmitter and enhances it with an error correction code. The enhancedinformation signal is output from the FEC coder 122 to the demultiplexer124. The demultiplexer distributes the information signal to the modemprocessors 126 a-126 n. The modem processors 126 a-126 n each formattheir respective portions of the enhanced signal for transmission.Outputs from the modem processors 126 a-126 n are summed by an adder 128and delivered to the transmission layer 130.

At the receiver 200, the physical layer 220 performs processing that isthe inverse of the processing that had been applied in the physicallayer 120 of the transmitter 100. The physical layer 220 may bepopulated by an FEC decoder 222, a multiplexer 224 and a plurality ofdemodulators 226 a-226 n. There will be one demodulator 226 a-226 n foreach of the traffic channels that had been allocated to carry theenhanced information signal. The recovered digital signal from thetransmission layer 230 is input to each of the demodulators 226 a-226 n.Each demodulator 226 a-226 n outputs a recovered portion of the enhancedinformation signal. The multiplexer 224 merges each of the recoveredportions of the enhanced information signal into a unitary recoveredenhanced information signal. The FEC decoder 222 performs errordetection and correction using error correction codes that had beenintroduced by the FEC coder 122 in the transmitter 100. The FEC decoder222 outputs a corrected information signal to the higher layers 210 ofthe receiver 200.

In a CDMA embodiment, which is shown in FIG. 1, a receiver 200 need notinclude an element corresponding to the adder 128 of the transmitter100; the demodulators 226 a-226 n each perform correlation. As is known,correlation permits a modem processor to discriminate a desired CDMAsignal from other CDMA signals that may be present in an aggregatereceived signal. Thus, in the embodiment of FIG. 1, the demodulators 226a-226 n identify and output respective portions of the recoveredenhanced information signal.

According to an embodiment of the present invention, the FEC coder 122and FEC decoder 222 may generate and decode iterative systematic nestedcodes, also known as “turbo” codes. These turbo codes provide anadvantage in that the FEC decoding process may be repeated iterativelyto improve the information signal recovered therefrom. Thus, the outputof a first iteration may be reintroduced to the FEC decoding block (pathnot shown in FIG. 1) for subsequent iterations. The nature of the turbocodes generates improved corrected data at subsequent iterations.

The known turbo codes, however, introduce a predetermined amount oflatency into the decoding process. Turbo codes operate on blocks of apredetermined size. For example, one turbo code being considered for awireless communication system for computer network applicationspossesses a block size of 4,096 channel symbols. An FEC decoder 222 mustdecode an entire block before a recovered information signal becomesavailable for the block. This characteristic may be contrasted withconvolutional codes which are used in the known IS-95 standard for CDMAcellular communication; convolutional codes are characterized byrelatively smaller latency for same-sized block (relative to turbocodes) because it is not necessary to receive an entire block beforedecoding may begin. It is believed that by distributing the FEC codeamong several parallel traffic channels as is shown in FIG. 1, thehigher aggregate throughput of the traffic channels ameliorates thelatency that would otherwise be introduced by the turbo code.

For high data rate applications using plural parallel traffic channels,it is believed that use of turbo codes achieves a higher figure of merit(lower E_(b)/N_(o) than for convolutional codes. Using the example of a4,096 channel symbol sized block and E_(b)/N_(o) of 1.5 dB the turbocode provides a BER of 10⁻⁶. By contrast, for voice systems requiring aless stringent 10⁻³BER, a convolutional code requires an E_(b)/N^(o) of7 dB or more.

The techniques of the present invention find application in a variety ofwireless communication systems including CDMA system. Typically, inapplication, the base stations and subscriber stations of the wirelesscommunication system will include functionality of both the transmitterand receiver of FIG. 1. That is, to provide two-way communication, abase station will include a transmitter portion 100 and a receiverportion 200. So, too, with the subscriber terminal. The base stationsand subscriber terminals may but need not be configured to providesimultaneous full-duplex communication.

Typically, a base station of a wireless communication system transmits aplurality of data signals to a variety of subscribers simultaneously.According to an embodiment of the present invention, each base stationmay perform the techniques disclosed herein simultaneously on a numberof high rate data signals. It is consistent with the spirit and scope ofthe present invention that each signal may have a date rate that isindependent of the data rates of the other signals. Thus, in such acase, a base station may be configured to include its FEC coder/decoders122, 222 and modem processor/demodulators 126 a, 226 a in a pooledconfiguration. Such an embodiment permits the base station to assign,for example, a variable number of modem processors 126 a-126 n to a datasignal depending upon the rate of the signal to be transmitted.Similarly, by including a pool of FEC coders 122 (shown singly in FIG.1)in a base station, the base station may selectively enable FEC coders122 as the base station receives new data signals to be transmitted tosubscribers. Provision of base station processing components in a pooledarrangement is well-known.

Several embodiments of the present invention are specificallyillustrated and described herein. However, it will be appreciated thatmodifications and variations of the present invention are covered by theabove teachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention.

I claim:
 1. In a CDMA (Code Division Multiple Access) transmitter, aphysical layer processor comprising: a source signal including data; anFEC (Forward Error Correction) coder to receive the source signal andproduce an enhanced source signal including data coded with errorcorrection information; a demultiplexer coupled to receive the enhancedsource signal from the FEC coder; a plurality of modem processors, eachof which is coupled to a unique output of the demultiplexer to processrespective portions of the enhanced source signal in independentchannels; an adder coupled to receive outputs of the modem processors toproduce an aggregate signal, the aggregate signal being a summation ofthe enhanced signal processed in independent channels; and a transmitterto transmit the aggregate signal over a carrier frequency.
 2. The CDMAtransmitter of claim 1 provided in a base station of a CDMAcommunication system.
 3. The CDMA transmitter of claim 1 provided in asubscriber station of a CDMA communication system.
 4. The CDMAtransmitter of claim 1, wherein the FEC coder operates according to aniterative systematic nested code.
 5. The CDMA transmitter of claim 1,wherein the FEC coder operates according to a turbo product code.
 6. TheCDMA transmitter of claim 1, wherein the FEC coder according to aconvolutional turbo code.
 7. An apparatus as in claim 1, wherein theaggregate signal is modulated onto a common carrier frequency.
 8. In aCDMA receiver, a physical layer processor comprising: a receiver thatreceives a wireless signal from a transmitter, the wireless signal beingformed at the transmitter by a summation of portions of a coded signalthat were processed in independent channels but were wirelesslytransmitted as a single aggregate signal; a plurality of demodulatorscoupled to receive an output of the receiver; and a multiplexer coupledto direct an output of the demodulators to an FEC (Forward ErrorCorrection) decoder to recover a single unitary information signal. 9.The CDMA receiver of claim 8 provided in a base station of a CDMAcommunication system.
 10. The CDMA receiver of claim 8 provided in asubscriber station of a CDMA communication system.
 11. The CDMA receiverof claim 8, wherein the FEC decoder operates according to an iterativesystematic nested code.
 12. The CDMA receiver of claim 8, wherein theFEC decoder operates according to a turbo product code.
 13. The CDMAreceiver of claim 8, wherein the FEC coder according to a convolutionalturbo code.
 14. An apparatus as in claim 8, wherein the aggregate signalis modulated onto a common carrier frequency.
 15. A physical layersignal processor for use in transmitting a Code Division Multiple Access(CDMA) encoded signal, the signal processor comprising: a Forward ErrorCorrection (FEC) encoder, connected to receive a source signal, and toapply an error correction code; a demultiplexer in communication withthe FEC encoder, the demultiplexer outputting two or more demultiplexedencoded signals; a plurality of modem processors, each receiving arespective one of the plurality of the demultiplexed encoded signals,the modem processors each modulating a respective one of thedemultiplexer outputs applied thereto to produce a respective one of aplurality of transmission code modulated signals, the signal processorfurther characterized by: an adder that is connected to receive theplurality of transmission code modulated signals to thereby produce anaggregate signal; and a transmitter connected to receive the aggregatesignal output by the adder, for producing an aggregate transmittedsignal.
 16. The processor of claim 15 provided in a base station of aCDMA communication system.
 17. The processor of claim 15 provided in asubscriber station of a CDMA communication system.
 18. The processor ofclaim 15, wherein the FEC encoder operates according to an iterativesystematic nested code.
 19. The processor of claim 15, wherein the FECencoder operates according to a turbo product code.
 20. The processor ofclaim 15, wherein the FEC encoder encodes according to a convolutionalturbo code.
 21. A method for transmitting a high data rate signal over awireless radio channel comprising the steps of: enhancing the high datarate signal with the Forward Error Correction (FEC) code; distributingthe enhanced high data rate signal over a plurality of demultiplexedsignals; encoding each of the plurality of demultiplexed signals with aCode Division Multiple Access (CDMA) transmission code; characterized bythe additional steps of: summing the plurality of CDMA transmissionencoded signals to produce an aggregate signal; and modulating theaggregate signal, to produce a transmitted signal.
 22. The method ofclaim 21 in which the transmitted signal is provided to a base stationof a CDMA communication system.
 23. The method of claim 21 in which thetransmitted signal is provided to a subscriber station of a CDMAcommunication system.
 24. The method of claim 21, wherein the FECdecoder operates according to an iterative systematic nested code. 25.The method of claim 21, wherein the FEC decoder operates according to aturbo product code.
 26. The method of claim 21, wherein the FEC decoderoperates according to a convolutional turbo code.